He didn't say why he left, but I speculate that it is because they cut 50% of his writing for PoE. He only wrote Durance and Grieving Mother and most of his content for them was cut. They were still better than everything else in the game even while unfinished. This isn't the first time he leaves a company.
Well, I have to disagree: I would say his companions were the more tiresome party members. We know this kind of characters since PT, but it's still sad to read, even if it was for the better.
@Hassat Hunter
I disagree, the SP worked quite well, but the game gained a lot of depth, when you played it in MP. Because every new game I started with another player felt different, even when I knew the game in and out. The mechanics are still the same, so I would say Larian isn't weakening the SP, they just put extrawork in the MP, that isn't necessary but appreciated.
@Octu
You forgot
Dragon Commander
.
Yes it is a bit confusing, but they wanted to create big sprawling worlds and games like Ultima

.
But seriously: If you take a look at the history of the company it kind of explains why they did, what they did.
Dragon Commander and
Original Sin were created to found
Divinity 3, until
Original Sin became a big project.
Beyond Divinity was only developed to rescue Larian from shutting down (that's why it's not
Divinity 2), after they got no money from
Divine Divinity. It's quite an interesting lecture if you read the developers diary in the Developers Cut.